1. Field of the Invention
This invention relates generally to the manufacture of fibers or filaments from thermosplastic materials, including vitrous thermoplastic materials such as glass.
More particularly, the invention relates to a spinnerette for making such fibers.
2. The Prior Art
Filaments of thermoplastic material, such as e.g. glass, are produced by maintaining a supply of the molten thermoplastic material in an extrusion crucible the bottom wall of which is provided with nozzles through which the molten material extrudes to form filaments. These are drawn off, for example by contact with a drawing drum rotating at high speed.
To permit continuous high-speed production of the filaments the extrusion crucible must be continuously supplied with additional molten thermoplastic material. For this purpose it is known to mount atop the extrusion crucible a melting crucible which forms part of the spinnerette and into which thermoplastic material is admitted in solid state, to become melted therein and to pass as a melt through openings in a dividing wall into the extrusion crucible. Clarification and homogenization of the melt takes place in the extrusion crucible as the newly admitted melt descends therein towards the nozzles in the bottom wall.
As the melt extrudes through the nozzles it turns to filaments which are drawn off by a suitable device, for example a drawing drum of the type disclosed in U.S. Pat. No. 3,676,096. The solidified filaments may be combined to form strands, or they may be severed to form fibers of desired length.
One way of feeding additional bodies of thermoplastic material to the melting crucible is disclosed in U.S. Pat. No. 3,489,542. There, spheres of glass are supplied along individual inclined paths into pre-melt chambers each of which can accommodate only one sphere at a time. In these chambers, which are located adjacent one another over the length of the melting crucible and which communicate with the same via restricted orifices, the spheres are preheated by radiation from the melting crucible until they soften and melt so that their material can pass through the orifices and drip into the melting crucible to unite with the contents of the same and form therewith a uniform melt.
The equipment required for this type of arrangement is necessarily rather complicated and expensive. Moreover, the bodies being fed must be of identical size to avoid malfunctions. If a malfunction does occur in the movement of such bodies along one of the paths, so that the supply to one of the pre-melt chambers (and hence to a portion of the melting crucible) is interrupted, this causes in that particular portion of the melting crucible a supply condition and a melt viscosity condition which differ from those prevailing in the other portions of the crucible; this influences the uniformity of the spinning (extruding) operation.
According to the disclosure of U.S. Pat. No. 3,615,314 the melting crucible which receives the solid thermoplastic material is not mounted directly atop the extrusion crucible, but is mounted in the sidewalls of the same which are upwardly extended for this purpose. The melting crucible is of triangular cross-section, with one corner of the triangle facing downwardly towards the spinnerette wall having the extrusion nozzles. Over its length the melting crucible is subdivided into a plurality of chambers which communicate with one another only in the region of the downwardly facing corner. Every other one of these chambers has an opening by which it communicates with the extrusion crucible; the remaining chambers have no such openings and receive the solid thermoplastic bodies to be melted.
Both of the two patents just discussed require uniformly dimensioned spheres or similar bodies for feeding of their melting crucibles. They cannot use supply bodies in form of pellets which are much easier to make (and hence less expensive) than the uniformly dimensioned bodies, but which are also much less uniform in size and shape. Such pellets are employed in German Published Application DE-OS No. 2,326,975 where they are dropped from above into the melting crucible by an inclined chute which moves back and forth over the length of the crucible.
All three aforementioned disclosures have in common that the melting crucible is not fed with solid supply bodies continuously and over its entire length or at least over a substantial continuous portion of its length. This means that the cold (or at least relatively cold) material of the supply bodies enters into the hot melt in the melting crucible at a plurality of locations which are spaced from one another lengthwise of the melting crucible. This in effect cools the melt at these locations whereas at the areas between these locations the melt is considerably hotter. Due to the communication of the melting crucible with the extrusion crucible, these same conditions will also prevail in the extrusion crucible so that the uniformity of melt temperature and melt viscosity which is required to obtain uniform spinning of the filaments, is lacking.
The temperature variations at the different melt crucible locations also cause non-uniform stressing of the crucible material which leads, particularly in areas subject to overheating, to a reduction of the spinnerette lifetime.
To assure proper spinning conditions it is necessary that the temperature of the melt in the crucibles (i.e., the melting crucible and the extrusion crucible) be as uniform as possible over the length of these crucibles. This is possible only if the interior of the melting crucible is not subdivided into chambers which, as in the current practice, either do not communicate with one another (and communicate only with the extrusion crucible), or which communicate with one another only via relatively small openings of restricted cross-section. Also, localized admission of supply bodies into the melt must be avoided if uniform melt temperature is to be obtained over the length of the melting crucible.
Still a further problem which is encountered in the type of equipment under discussion, is the fact that spinnerettes must be made of platinum or of a platinum alloy. Such spinnerettes are relatively long in direction lengthwise of their crucibles and relatively narrow in their width; a typical example is a length of 900 mm and a width of 70 mm, not including the refractory material in which the spinnerette is mounted. Because of the high cost of platinum or platinum alloys the spinnerette walls are made very thin, usually having a thickness less than 1 mm. Due to the high temperature fluctuations which occur in the operation of this equipment (i.e., at start-up and at shut-down), these walls are subject to quite extreme stresses. This presents a serious problem, because the walls are made so thin to reduce the amount of expensive platinum or platinum alloy that is required, whereas on the other hand such thin walls often prove to be insufficiently resistant to deformation resulting from the thermal stresses. Thus, it may (and does) happen that during shut-down (i.e., cooling-off) the walls buckle inwardly towards one another; this changes the volumetric content of the crucible, may lead to the formation of cracks or fissures and causes difficulties in the admission of supply bodies.
Of course, deformation resistance of the walls can be increased by providing cross-braces, as for example disclosed in U.S. Pat. No. 3,056,846. However, this proposal leads to the use of additional expensive platinum or platinum alloy and such cross-braces tend to obstruct the free flow of the melt.